Heat pump systems are conventionally known to heat a heating target fluid by heat pump cycles (vapor-compression refrigeration cycles). In the heat pump cycle applied to this kind of system, an exterior heat exchanger serves as an evaporator that evaporates a low-pressure refrigerant by exchanging heat with the outside air. When a refrigerant evaporation temperature at the exterior heat exchanger is decreased to 0° C. or lower, frost formation might occur in the exterior heat exchanger.
Further, the so-called hot gas defrosting is also known as a means for removing frost formed in the exterior heat exchanger in this way. The hot gas defrosting involves allowing a high-temperature, high-pressure refrigerant (hot gas) discharged out of a compressor in the heat pump cycle to flow into the exterior heat exchanger, thereby defrosting the heat exchanger. However, it takes a relatively long time to carry out hot gas defrosting, which might increase the energy consumed by the compressor for defrosting.
For this reason, Patent Document 1 discloses a heat pump system for use in a vehicle air conditioner. The heat pump system is designed to defrost the exterior heat exchanger with frost formation using waste heat as a heat source, which is stored in a coolant for cooling vehicle-mounted electric devices. In the heat pump system disclosed in Patent Document 1, the exterior heat exchanger can be defrosted using the waste heat from the vehicle-mounted electric devices, thereby suppressing an increase in energy consumption by the compressor for defrosting.